Bis(4-hydroxyaryl)alkanes, which are the condensation products of a phenol compound with a carbonyl compound in the presence of an acid catalyst, are used in the manufacturing of many commercial products, such as polycarbonates and epoxy resins. Of particular technical and commercial importance is the condensation product of phenol and acetone, 2,2-bis(4-hydroxyphenyl)-propane, often called bisphenol-A. The world production of bisphenol-A in 1999 was over 2 million metric tons per year and is still growing. A very high purity of bisphenol-A is essential for the quality of the commercial products to which it is processed, such as polycarbonates. Well-known procedures for producing bisphenol-A include the steps of i) reacting an excess of phenol with acetone in the presence of an acidic catalyst to produce a product mixture containing bisphenol A; ii) passing the product mixture to a crystallizer to produce a suspension comprising a crystalline adduct of Bisphenol A and phenol, iii) separating the adduct from the mother liquor and washing the adduct crystals, and iv) distilling off the phenol from the bisphenol A:phenol adduct to obtain bisphenol A. The reaction mixture typically contains bisphenol A, non-reacted phenol, water, usually also some non-reacted acetone, but also some by-products, such as 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (o,p-isomer), 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman, polyphenols with three or more phenyl rings in the molecule, substituted indenes, hydroxyphenyl-chromanes, hydroxyphenyl-indanoles, or substituted xanthenes. These by-products cause an undesirable color to the bisphenol A:phenol adduct unless they are effectively separated from the crystalline bisphenol A:phenol adduct.
Accordingly, the skilled artisans have spent much efforts on the crystallization of bisphenol A:phenol adduct, the separation of the crystals from the mother liquor and on the purification of the bisphenol A:phenol adduct.
Numerous patent applications have been filed on the crystallization step, for example U.S. Pat. Nos. 4,740,635; 4,861,919; 4,927,973; 5,345,000 and 5,545,764. Crystallization in the presence of water, multistage crystallization and in-process crystallization are described in these publications.
WO 01/46105 A1 addresses the problem of separating the adduct crystals from the mother liquor. It suggests a process wherein the adduct crystals are separated from the mother liquor by means of continuous filtration in a rotating vacuum drum containing several filter cells and are subsequently washed with phenol and drawn off from the washing liquor by suction.
Other publications mainly relate to the washing of the adduct crystals. U.S. Pat. No. 5,434,316 discloses that the crystals are washed with hot water.
In view of the large amount of bis(4-hydroxyaryl)alkanes used in commercial production and the high requirements for their purity, it is still desirable to find a new process for recovering adducts of a bis(4-hydroxyaryl)alkane and a phenolic compound and for recovering bis(4-hydroxyaryl)alkanes. It is particularly desirable to find a new process wherein the mentioned adducts and bis(4-hydroxyaryl)alkanes of very high purity can be recovered.